1. Field of the Invention
The present invention relates to a method for determining refractive index distribution of, for instance, a cylindrical glass rod used as rod lenses or a preform for use in making optical fibers as well as an apparatus for determining such refractive index distribution.
2. Background Prior Art
The refractive index of cylindrical glass rod used as preforms (base materials) for making optical fibers or rod lenses per se varies radially is distributed as a square distribution along their radial direction while remaining unchanged in the axial direction. Optical fibers can be obtained by drawing such a cylindrical glass rod. Therefore, it is quite important to correctly determine the refractive index distribution of the preform prior to drawing it into optical fibers, so that products having high quality would be obtained.
The principle of the method for determining refractive index distribution of a cylindrical glass rod is explained below referring to FIG. 3. In FIG. 3, reference numeral 10 represents a cylindrical glass rod. The reference numeral 27 represents a screen to which ray "b" of light incident upon the cylindrical glass rod 10 is perpendicular. The x-coordinate and y-coordinate are defined as origin "c" which is a point projected onto screen 27 by rectilinearly propagating the incident ray "b". When the cylindrical glass rod 10 is placed as traced with 2-point chain line, incident ray "b" enters the center of the cylindrical glass rod 10 and then passes rectilinearly through as outgoing ray "d.sub.0 ", which projects at origin "c" on screen 27. Then if the cylindrical glass rod 10 is displaced in parallel from the place traced with the 2-point chain line to a place traced by the continuous line, the incident ray "b" passes through the cylindrical glass rod 10 refracting under refractive index distribution thereof and emerges from the cylindrical glass rod 10 as outgoing ray "d.sub.1 ", which projects at point Xc of x-coordinate on screen 27. The point Xc is the displacement coordinate by displacing distance "t" of the cylindrical glass rod 10, thus is represented as a function of "t" as in the following formula: EQU Xc=f(t)
Outgoing angle .PHI. (t) has the following relation: EQU .PHI.(t)=tan.sup.-1 (Xc/L)=tan.sup.-1 {f(t)/L}
In this formula, "L" represents the length from the center of the cylindrical glass rod 10 to the screen 27. The outgoing .PHI. (t) thus can be determined from measuring results of the points Xc by the displacing distances "t" of the cylindrical glass rod 10.
Then the refractive index distribution n(r) of the cylindrical glass rod is calculated from the following formula, using the angle .PHI. (t) of outgoing ray thus obtained: ##EQU1## In this formula, "n.sub.2 " represents the refractive index of the surrounding area of the cylindrical glass rod 10 and "a" is the radius of the cylindrical glass rod 10.
Japanese Patent Provisional Publication Nos. 63-95336 and 63-95337 disclose methods for determining refractive index distribution of a cylindrical glass rod by using the above explained principle.
In the case where a cylindrical glass rod is prepared by, for instance, growing glass along its axial direction according to the VAD (Vapor-phase Axial Deposition) technique, layer structure may be formed within the growing glass and the layer structure often makes refractive index distribution of the resulting glass heterogeneous in the axial direction. Point "e(x,y)" in FIG. 3 shows a projected point of outgoing ray "d.sub.e " on screen 27 from cylindrical glass rod 10 having striae deposited at the place traced with the continuous line. Incident ray "b" passes through the cylindrical glass rod 10 diffracting by the layer structure, the outgoing ray "d.sub.e " disperses in the direction of the y-axis, and then is projected at the point "e(x,y)" far from the x-axis on the screen 27.
Conventionally, even if a cylindrical glass rod had layer structure and an outgoing ray was projected at a point "e" far from the x-axis, outgoing angle .PHI. was determined from only the x-coordinate of the point "e". Thus, it is difficult to correctly determine the angle .PHI. of the outgoing ray of the cylindrical glass rod having great deal of layer structure, the result being, that this causes determined values to be errors. If the refractive index distribution is calculated from such erroneous values of the angle .PHI. of the outgoing ray, the resulting refractive index distribution causes great variation in the portions of the cylindrical glass rod within which great deal of layer structure is present and thus a correct refractive index distribution cannot be obtained.